Catalysts

ABSTRACT

This invention relates to a method of preparing iron-promoted catalysts in a pelletized form for the synthesis of ammonia which comprises the steps of fusing a mixture consisting essentially of iron oxides of the approximate composition of magnetite together with additional oxides of promoter metals in a total proportion of the oxides of promoter metals of from about 1 percent to about 10 percent, rapidly cooling the fusion product, grinding the fusion product to give fine, regular granules, reducing substantially completely the fused granules, coarsely grinding the reduced fused granules, agglomerating the coarsely ground product into pellets by simple compression and recovering said catalyst in a pelletized form. The invention also relates to the catalysts so produced.

D United States Patent [151 3,644,21 6 Egalon et al. 1 Feb. 22, 1972[54] CATALYSTS 1,618,004 2/1927 Greathouse ..252/472 [72] lnventors:Roger Egalon; Ramiro Tella, both of Saint gig '3 3/1966 i "'252/455Andre Nord France 3 12/1968 Hmnchs ..252/472 1,853,771 4/1932 Larson..252/477 X [73] Assignee: Societe Anonyme: Ugine Kuhlmann, Paris,

France Primary ExaminerDaniel E. Wyman [22] F'led: 1969 AssistantExaminer-Philip M. French 21 APPL 51 434 Att0rneyl-lammond&LittellRelated US. Application Data 57 ABS CT [63] Continuation-impart of Ser.No. 705,862, Feb. 15, I

1968, ab do d, c ti ti q n f s N This invention relates to a method ofpreparing iron-promoted 386,481, July 22, 1964, b d catalysts in apelletized form for the synthesis of ammonia which comprises the stepsof fusing a mixture consisting essen- [30] Foreign Application PriorityD t tially of iron oxides of the approximate composition of magnetitetogether with additional oxides of promoter metals in a July 24, Francetotal proportion of the oxides of promoter metals of from about 1percent to about 10 percent, rapidly cooling the fu- [52] US. Cl...252/455 R, 252/466, sion product, grinding the fusion product to givefine, regular [51] Int Cl B0 11/40 granules, reducing substantiallycompletely the fused [58] Field 0. 42, 5 472 granules, coarsely grindingthe reduced fused granules, 1 2 l glomerating the coarsely groundproduct into pellets by simple compression and recovering said catalystin a pelletized form. 5 6] Reerences Cited The invention also relates tothe catalysts so produced.

7 STATES PATENTS 7 Claims, No Drawings 1,698,300 1/1929 Ehlers ..264/1111,489,497 4/ 1924 Carson .252/466 CATALYSTS CROSS-REFERENCE TO OTHERAPPLICATIONS This application is a continuation-in-part of our copendingapplication Ser. No. 705,862, filed Feb. 15, 1968, now abandoned, whichin turn was a streamlined" continuation application of Ser. No. 386,481,filed July 22, 1964, now abandoned.

THE PRIOR ART It is known that the catalytically active principle forthe synthesis of ammonia is microcrystalline iron. The only practicalway of obtaining said catalytically reactive iron is that which consistsin starting from Fe O (magnetite), and in reducing said oxide to a-iron.Preferably the Fe O, (magnetite) prior to reduction contains in solidsolution small quantities of refractory oxides known as promoters, suchas A1 CaO, K 0, etc. These oxides promote the formation of a-ironmicrocrystals and electronically enhance the catalytic properties of thesurface of the iron crystals. In the course of this reduction, which isgenerally carried out by means of hydrogen or synthesis gas comprisingessentially a mixture of suitable proportions of nitrogen and hydrogen,the release of oxygen atoms in the form of water vapor forms a finenetwork of micropores inside the mass in contact with the heatedreduction phase. It has been shown that promoters such as alumina, CaO,MgO, SiO etc., maintain this microporous texture while avoiding thermalfritting of the iron microcrystals formed and their recrystallizationinto large crystals, whereas this recrystallization is promoted by thewater vapor formed during the reduction. The electronically activepromoters are such oxides as K 0, Cs O, etc.

In order to obviate this undesirable action of the water vapor, it isnecessary to operate under conditions such that the gaseous flow sweepsat a high space velocity over all points of the catalytic surface beingformed, in order not to allow the water vapor resulting from thereduction to stagnate locally. In

order to carry out the reduction satisfactorily, it is thereforenecessary for the crude catalyst to be previously subdivided intogranules of regular geometrical shape, having good mechanical strengthand a homogeneous physicochemical texture, while allowing them to retaingreat reducibility at the lowest possible temperature.

It is also known that the fusion product of magnetite and promoteroxides undergoes segregation after pouring, due to migration inconnection with the different rates of cooling of its moltenconstituents. Moreover, the grinding and subsequent screening of thecooled mass leads to solid particles or fragments of various geometricalshapes and of heterogeneous sizes. In order to impart to these fragmentsan adequate mechanical strength after reduction, it is necessary to addto the magnetite before fusion substantial proportions of lime andalumina, which further increases the above-mentioned disadvantages,namely, segregation on cooling, a heterogeneous texture of the crudefragments, and the difficulty of reducing them in a satisfactory manner.

Numerous attempts have been made to obviate these disadvantages incatalysts based on an a-iron utilized up to the present time in thesynthesis of ammonia.

According to the prior practice a dispersion as homogeneous as possibleof the promoter additives in the magnetite was aimed at. This wasobtained by incorporating the additives in the molten magnetite and byrapidly cooling the mixture to limit the unfavorable phenomena ofmigration of the additives and thus the heterogeneity of the activatedmagnetite or raw catalyst." The raw catalyst was then broken into smallpieces of irregular shape, which were sieved to obtain batches having avarying, but not too widely spread, particle size.

Starting from this stage, two ways were followed to obtain the activecatalyst.

The more generally used was the reduction in situ of the raw catalyst,i.e., the reduction thereof in the synthesis reactor tubes. Numerousstudies and publications have been made about the conditions of such areduction. All recommend a relatively low reduction speed at atemperature of the order of 500 C. These conditions, which lead to highexpenses, result from the fact that the reduction of magnetite to ironis accompanied by two major facts:

-the change of crystalline system -the emission of water.

Magnetite crystallizes in the cubic system, whereas when crystallizes inthe face-centered cubic system. It luckily happens that the dimensionsof the respective crystalline system are very close to one another.There is thus no variation in volume.

However, a complete crystalline reorganization takes place once theoxygen has been displaced. The disappearance of oxygen leads to a 27percent reduction of weight and therefore of the specific gravity of thecatalyst.

The reduction of the catalyst gives the reaction:

It is well known that when the water which is formed remains in contactwith the catalyst during reduction it affects the formation of a-ironmicrocrystals and thus diminishes the reactivity of the final catalyst.

The practical consequences of these two phenomena are that, on the onehand, the reduction process should be carried out very slowly so as tosweep of? the water as it forms, said water passing through thedownstream catalyst bed, and on the other hand, the crystallinemodification is accompanied by a structural weakening, and, therefore,by a partial loss of mechanical properties, and by the formation ofcatalyst dust which must be eliminated as soon as possible to avoidabrasion of the circulation pumps of the synthesis circuit.

in order to avoid such drawbacks, it has appeared as logical to take asecond way, that is, the prereduction of the catalyst in conditionswhich are as closely controlled as possible. The reactivity of thecatalyst can thereby be improved, and the formation of dust in theoperating circuit is lessened. On the other hand, this second method hastwo inconveniences with respect to the first. First, a surfacereoxidation must be carried out at the end of the reduction in orderthat the catalyst may be stored and manipulated in air (antipyrophorictreatment). Second, from the psychological point of view, theprereduction entails an increase of the selling price of the catalyst,whereas the actual cost of the reduction in situ is often drowned in thegeneral costs of the synthesis plant.

Finally, the two above-described ways, l) reduction in situ and (2)prereduction, lead to the same type of catalyst, a basically a-ironactivated catalyst in the form of small pieces of irregular shape andlow mechanical resistance.

A third process is also known for the improvement of these catalysts.This involves finely crushing the crude fused oxides, pelletizing themin the oxidic state, generally in the presence of a binder or alubricant, in order to confer a certain amount of cohesion to the powderand thereafter, by metalloceramic techniques, subjecting the material toa fritting at high temperatures to improve the mechanical stability oftheoxidic catalyst. Thereafter the fritted material is fragmented andreduced, either in situ or as a separate step. Unfortunately, thegranules obtained from this crude powder do not acquire sufficientphysical strength unless a binder or a flux is previously added to thecrude ground powder, or unless the fragile crude granules are brought toa sintering temperature which is dangerously close to the meltingtemperature.

Apart from the cost of such a pelletizing, due to the hardness of thestarting material, the sintering steps by superficially melting themagnetite leads away from the obtention of a crystalline material withan area as great as possible. Finally and mainly, the subsequentreduction of the pelletized catalyst has exactly the same inconveniencesas the conventional reductions. Only the mechanical conditions of shapeand resistance are, therefore, improved, at the price of a costlytreatment and of a loss of reactivity.

OBJECTS OF THE INVENTION An object of the present invention is theobtention of a highly reactive catalyst which has good mechanicalstrength, is prereduced, and is in the form of pellets.

Another object of the invention is the development of a method ofpreparing iron-promoted catalysts in a pelletized form for the synthesisof ammonia which comprises the consecutive steps of l) fusing a mixtureconsisting essentially of iron oxides of the approximate composition ofmagnetite, together with additional oxides of promoter metals in a totalproportion of the oxides of promoter metals of from about 1 percent toabout 10 percent, (2) rapidly cooling the fusion product, (3) grindingthe fusion product to obtain fine regular granules, (4) reducingsubstantially completely the fused granules, (5) agglomerating thecoarsely ground reduced product into pellets by simple compression, and(6) recovering said iron-promoted reduced catalyst in a pelletized form.

These and other objects of the present invention will become moreapparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION For the purpose of achieving the pelletedcatalyst according to the invention which has a basis of activated,reduced iron and is intended for the synthesis of ammonia, a crudecatalyst is first prepared by the known method by melting a mixture ofiron oxides of the approximate composition of magnetite, Fe O,, togetherwith small amounts of oxides of promoter metals, as conventionally used.These amounts in total vary from about 1 percent to about 10 percent ofthe mixture of oxides. Preferably Al O is present as one of the oxidesof promoter metals due to its refractory properties. The A1 0 should bepresent in an amount of from 0.5 to 9.0 percent, preferably from 1.0 to2.5 percent, of the total weight of the mixture of oxides. In addition,it is also preferable to include K 0 as one of the oxides of thepromoter metals due to its electronic promotive activity. The K 0 shouldbe present in an amount of from 0.075 to 2 percent, preferably from 0.1to 0.5 percent, of the total weight of the mixture of oxides. Otheroxides of promoter metals, such as SiO alkali metals and earth alkalimetals oxides, etc., may also be included in the mixture of oxides to befused. In this respect, lithium oxide corresponding to from 0.1 to 0.5percent of Li 0 may be added. However, the presence of these otheroxides are not necessary to the practice of the invention.

The fused mixture of oxides is then rapidly cooled to limit theunfavorable phenomena of additive migration as is conventional.Preferably the fused mixture is poured in a thin layer on a movingcooled mold. The cooled mass is then ground into fine, regular granulesor fragments of from about 2 to 10 mm. The resulting fragments aresubjected to reduction, preferably a practically complete reduction, inview of the subsequent pelletizing. Preferably the reduction isconducted with small volumes of the fragments as compared with the largevolumes of catalysts previously reduced in commercial synthesis tubes.This reduction can be made in thin layers of the fragments utilizing agas containing hydrogen as the reducing agent. Preferably it isadvisable to utilize a synthesis gas having a ratio of 3I-I to N,. Thetemperature of the reduction may be between ambient temperature and 550C.,.

preferably around 500 C., at ordinary or slightly elevated pressure. Theincrease in temperature programming of the reduction together with thethroughput of the reducing gases should be selected in order that thewater content of the exit gases preferably, but not limitingly, notexceed 1 percent, preferably 0.25 percent. The reduction should becontinued until no further water vapor is found in the exit gases attemperatures of over 500 C.

A friable reduced product is obtained by the reduction. The product isthen ground coarsely to give a microcrystalline airon powder, preferablypassing through a 2-mm. mesh or finer sieve. It is also possible toreduce the granules in a fluidized bed. In this case, it is preferableto grind the cooled fused mass to fine, regular granules of less than 2mm. When reducing these granules in a fluidized bed, the water contentof the exit gases is not as critical. The same type of reducedmicrocrystalline a-iron powder is obtained. The reduced,microcrystalline powder is then formed into pellets, even withoutaddition of adjuvants. The pelleting at this point takes advantage ofthe malleability of the a-iron contentof the reduced powder. Thepressures required for the necessary compression of the pellets variesaccording to the type of pelletizing presses, size of the pellets andthe quality of the reduced microcrystalline powder. Pressures in theorder of 1,000 to 10,000 kg./cm. may be employed. Higher pressures maybe employed but the resulting pellets have a somewhat lower catalyticeffectiveness.

It has been found that by subjecting the granules of crude catalystobtained as described above to a high degree of reduction, and under thespecial conditions which have just been mentioned, extremely friablereduced catalyst granules are obtained which by coarse grinding yield amicrocrystalline powder capable of being agglomerated into pelletspossessing good mechanical strength by simple compression, without theaddition of a binder, flux, or any solid or liquid adjuvant, thesepellets constituting the advantageous form of the catalyst according tothe invention.

It was also found that even if the rate of compression of theabove-mentioned microcrystalline powder varies within wide limits, forexample, from 1 to 15 tons per square cm., the microporosity of theresulting pellets remains constant and very high, as is proved byspecific surface measurements.

In addition, the macroporosity of the resulting pellets is likewise highand can be controlled, because it is inversely proportional to the rateof compression applied within the limits indicated above.

It was totally unexpected to find that the compression of the reducedmicrocrystalline powder into pellets did not destroy the crystallinestructure of the a-iron crystals, and more particularly, the microporeswhich are the seat of the catalytic phenomena.

It, therefore, becomes perfectly possible to manufacture pellets ofprereduced catalyst having an excellent macro and microporosity. Thisoperation leads to a very important increase of specific gravity of thecatalyst without the catalyst showing a substantial loss of reactivity.

The pelletizing step can be operated in air with a reducedmicrocrystalline powder previously submitted to an antipyrophorictreatment, although the grinding and pelletizing of the prereducedfragments can be conducted in an inert atmosphere. This is veryimportant since it could be expected that the superficial formation ofmagnetite with the antipyrophoric treatment would interfere with thefurther operations of crushing and pelletizing. The antipyrophorictreatment can be conducted at about room temperature with a reducing gascontaining from 0.05 to 0.1 percent of air or proportionately lesseramounts of oxygen.

It is, therefore, possible by the present invention to obtain theoptimal composition of the catalyst with respect to its electronicallypromoting oxides without regard to any possible loss of mechanicalproperties which might occur on reduction.

The pellets of catalyst obtained by the process of the invention aremechanically stable even when low amounts of refractory oxides such asA1 0 CaO and SiO; are utilized. The catalyst pellets are preferablyemployed as such and directly charged into the reactor tubes for ammoniasynthesis. However, they may be crushed and sized if smaller catalystfragments are desired. Tests have proven that in either event the amountof dust production from the catalysts by erosion of the gases passingthrough the synthesis reactors is practically nil.

Where catalysts are involved, the interest of the invention lies in thetechnical performances achieved thereby.

a. The catalyst of the invention has an apparent specific gravity atleast 15 percent higher than that of any of the ironbased catalystsknown hitherto. This comparison is, of course, based on the active formof the catalyst, i.e., after reduction.

This increase of the specific gravity makes it possible to charge, in agiven conventional reactor, percent more catalyst, or conversely toreduce by 15 percent the volume of a new reactor. This fact is veryimportant at a time when the position of magnetite and promoters havingthe following composition:

Amount of Fe" in the total mixture of oxides 22.36% Ai O5axl .99%

size of the reactors steadily increases. 5 K O=% b. The catalyst of theinvention in the form of regular-sized Li O=% pellets makes it possibleto decrease very substantially the loss After being crushed, granulatedinto particles of about 3 of pressure of the synthesis fluid flow duringits passage mm, and disposed in a thin layer in a suitable converter,this through the catalyst bed. Furthermore, the preferential fusionproduct was reduced by the synthesis mixture N +3H passages through thebed are considerably reduced. 10 which was passed at high space velocityand at an initial temc. in spite of the pelletizing, and due to the factthat perature of 150 C., the space velocity and the temperaturemicrograins are agglomerated, the catalyst of the invention being soregulated that the water vapor content of the gaseous has amacroporosity which is five times greater than the prior reducing phasedid not exceed 0.25 percent. The end of the art catalyst in brokenpieces or fragments, thereby substanreduction was carefully checked bygravimetry in order to tially increasing the accessibility to the activesites. 5 m ke sur hat it w compl Thi w n by m ans of d. The mechanicalstrength of the catalyst of the invention is differential weighings ofascarite tubes traversed by the gaseconsiderably increased. in practice,and contrary to what hapous phase and retaining quantitatively the waterto be deterpens with prior art catalysts in broken pieces or fragments,the a mined. The temperature of the mass was then approximately loss ofpressure through the catalyst bed does not increase 500 C. The'mass wasthen cooled and treated by the same with time and there is no formationof dust. I mixture N +3H additionally charged with 1 percent of air.

e. Due to the possibility of achieving the optimum composi- The rise intemperature of the mass during this operating did tion of the catalyst,reactivity levels have been reached which not exceed 30 C., 'a'nditstermination was marked by a return could not be obtained heretofore. Forexample, the converto ambient temperature. sion rate is 22.i percent at460 C. under a pressure of 250 it was then possible for the mass to beeasily ground into a bars with a space velocity of 25,000. coarsepowder, which it was easy to agglomerate into pellets As indicated,after their reduction and cooling, the granules of a diameter'of 10 mm.by simple compression at a pressure may advantageously be treated inknown manner by the gas or S t 10 t n p cmF, without any adjuvant, in aconventional gaseous mixture previously used for the reduction butcharged pelleting device equipped with a pressure-indicating stabilizer.with a Small proportion P i0 1 P Of The microcrystalline massessentially made of a-iron is pale this additional treatment having theeffect of eliminating any he th t f macropore's is dark andhomogeneously Py p y 0f the granulesby Covering them with a Shh! ofdistributed in the catalyst in the proportion of to percent Oxide- Thisamiipyl'ophol'i.c treatment y 3150 hi? made on the of the total volume.The pellets obtained as above described catalysts after pelletizingor,-if the pellets are to be utilized imware ground and i d to b i a h if 1 to 2 mm, A mediately, it can be Omitted 35 layer of 22 cc. of theabovedescribed ground and sized pellets ThePehets which constitute theCatalyst, according to the used as catalyst for the synthesis of ammoniain a vertical tumvenhon, Porous homogeneous geometrically identical-bular laboratoryreactor of 25 mm. diameter, under different hmechahlcahy yery Q AS is shown by Pbservahohs conditions of temperature,pressure and space velocity of the with h mlcroscope and the mercul'ypohoslmetfhi they synthesis gas had a reactivity the respective valuesof which cohta'h a homogeneous network of mactopore? Ofa hamster 40 aresummarized in Table l below. Reactivity is indicated by ehual to or hthan 2000 ahgshomsg which a the percentage content of NH contained inthe gas leaving the Shel-able sheclhc 'h and (hsmhhmd h K reactor. Thetemperatures indicated weretaken at the hottest microcrystalhne mass ofa -iron constitutmgthe internal active v poinfof the catalyst layersurface of the pellets. Tins structure permits easy access for I v thereacting gaseous phase to all points of the inside surfaceof TABLE! thecatalyst, while the high apparent density and the regularity I g I spaceMucky Reacmily of shape of the pellets permit a filling of greaterweight of th Temperature Pressure VoL/voL/hr. as NH, synthesisconverter, for an equal volume, without increasing the pressure drop,and the elimination of preferential passages C [-324 m mom 311% for thecurrent of gas in the charged converter, thus ensuring 5O Me ci '324bars 28,200 28.1% more effective utilization of the charge duringoperation and 360C- 212% providing increased productivity. A

The following examples are given for the purpose of illus- Table I]below compares the physical properties of catalyst trating thehlvehfiohare not, however, t0 be deemed pellets according to theinvention, having a diameter of i0 hmitahve in y p I 1 mm. and obtainedby compression at respective pressures of 3 EXAM El and 10 tons perpellet (about 3.8 tons/cm. and 13 tons/cm?) with those of fragmentsobtained by crushing, screening, and

A crude catalyst was first prepared by very quickly cooling sizing theconventional catalyst, which is then reduced under in a thin layer, on amovable, cooled ingot mold, the fusion identical conditions to those ofthe reduction of the catalyst product of a mixture of oxides of iron ofthe approximate com pellets forming the subject of the invention.

TABLE Ii Fragments Pellets according to the invention Dimensions 10x6 mmDiameter 10 Diameter 10.2 mm.; height mm; height 6.4 mm. 11.1 min. Realdensity 3.80 3.68 3.72. Apparent density 2.02 2.25; compression 2.29;compression i w v 3 tons/pellet. 10 tons/pellet. Macroporosity 22,000 A0.01 cmfi/g 0.062 cmfi/g 0.026 cmJ/g.

Mean porosity between 400 A. 0.095 cm. /g 0.091 cmfi/g 0.09 emfi/g.

and 2.000 A. Microporosity expressedby 18 m. /g 17.5 mfl/g 17.2 mfl/g.

the BET surface oi'the pores of a diameter smaller than 400 A.

Fragility Fragments erush- Very hard, not breaking into fragments ableby hand when freely dropped onto tiles from without effort. 7 a heightof 10 meters.

NIT-' content in the gas leaving the reactor after synthesis at a spacevelocity of 16,000 at 446 C. and at 324 bars.

EXAMPLES II to VI A series of crude catalysts were first prepared byquickly cooling molten masses of mixtures of oxides with magnetitehaving the following compositions of promoter oxides as shown in TablelIl,

TABLE III This table demonstrates that the mechanical strength of thepellets is great, even where the total amount of refractory oxides islowest. As a comparison, conventional 3 to 8 mm. fragments of a catalystcomposition identical to that of Example Ill, reduced but not pelletizedgave 12.5 percent of dust when subjected to the same abrasion test. Thissize fragment is ordinarily utilized in industrial reactors.

Furthermore, industrial experience with several reactors of varioustypes, has shown that the pellets having an abrasion rate (measured inthe apparatus mentioned) lower than 6 per- .cent, had sufficientmechanical properties to resist erosion by gas under pressure.

The macropore volume for the catalyst composition of Example III wasdetermined before and after pelletizing at a pres- Promoter oxidecontent of the Total percent crude catalyst fpromoler sure of 5,800kg./cm. The 3 to 8 mm. fragments of the oxides prereduced catalyst werecompared with the same material t r' di owde a in throu ha l-mm. meshand sio2 CaO MgO A120, K20 SiO +CaO af er {5 gtoap rp g g (PCP (pep (pep(pep (pep Mgo A1201 pelletizmg and the results are glven in Table V.Examples Cent) cent) cent) cent) cent) 7 TABLE V 0.22 0.54 0.50 0.481.45

.31 .50 1.92 .50 3.02 .18 .46 4.31 .52 5.13 2.72 .35 3.53 .5l 7.52 Pmmy8- 24 A7 8.70 A5 Size of Pores in A l0 to l0 l0 to 7.5Xl0

After crushing, the different catalysts in 3 to 8 mm. frag- P'mdmdfragments were subjected to reduction with the mixture N +3H at a spacevelocity of 4,000 with a heating program of 5 C. per hour from ambienttemperature to 520 C. Reduction was continued at 520 C. for 10 hours.

After cooling to ambient temperature, they were treated with a stream ofN +3H containing 400 p.p.m. of oxygen until their pyrophon'citydisappeared.

The prereduced, treated fragments were then crushed into a powderpassing through a l-mm. mesh. This powder was pelletized in adouble-action rotary press at a pressure of 5,800 kg./cm. withoutaddition of any adjuvants.

The pellets of reduced catalyst had a diameter of 5 mm. and a length of5 mm. They were subjected to identical mechanical tests so as to showthe correlation between the total proportion of refractory promoters andthe mechanical properties.

Resistance to shearing was measured on the middle portion of the pellet(the most fragile portion) by means of a balance, one arm of which isprovided with a shearing knife, the shearing force being applied on theother arm.

The abrasion rate (percent of dust) was obtained by rolling the pelletsin cylinders (in the direction of the generatrices) of a devicespecially made for this purpose.

The test lasted for 1 hour at a speed of 10 r.p.m.

This test is very stiff with respect to the abrasion which pelletsundergo both during transport and in the reactor. Table IV gives theresults of these mechanical tests.

ments size 3 to 8 mm. (apparent density 3.8) Prereduced pellets dia.=5mm., l=5 mm. (apparent density 4.!)

The porosity of pellets between 1,000 and 75,000 A. is therefore 4.2times greater than that of fragments from which the powder, which waspelletized, was obtained.

As a further comparison, the catalyst composition of Example III wasfurther treated in various manners as follows:

a. The 3 to 8 mm. fragments in the nonreduced state were ground andsized to obtain a mesh size of 0.4 to 0.5 mm.

b. The 3 to 8 mm. fragments in the nonreduced state were reduced, groundand pelletized according to the invention. Thereafter, the pellets wereground and sized to obtain l. a mesh size of0.4 to 0.5 mm.

2. a mesh size of2 to 3.1 mm.

c. The 3 to 8 mm. fragments in the nonreduced state were reduced, groundand sized to obtain a mesh size of 2 to 3.1 mm.

d. The 3 to 8 mm. fragments in the nonreduced state were finely groundto powder. From said powder one-third passing a 20-mesh sieve andtwo-thirds passing a IOO-mesh sieve were thoroughly mixed. This mixedpowder was pelletized. The unreduced pellets were sintered at aboutl,l20 C. for several hours giving a pellet of good mechanical strength.These sintered, unreduced pellets of catalyst were then ground and sizedto a mesh size of 0.4 to 0.5 mm.

As indicated above, before being charged in the ammonia synthesisreactor of Example I (laboratory reactor) the precatalyst or catalyst,according to the case, whether just crushed after fusing, pelletizedaccording to the invention or sintered, was physically reduced toparticles having a grainsize of:

l. 0.4 to 0.5 mm. in order to eliminate the limitations due to gaseousdiffusion. (Recent experiments reported by Nielsen in Journal ofCatalysis, Vol. 3, Feb. 1964 show that such limitations are avoided withgrains smaller than 0.7 mm.) The space 2. 2 to 3.1 mm. in order to takeinto account the said gase ous diffusion phenomenon and evaluate theutility of the macropores obtained by pelletizing. The space velocitywas The results of these comparative tests are given in Table VI. Ineach instance the catalyst charged was thoroughly reduced 1 before theactivity was measured under comparable condigreater (one-fourth thepercent of i U V The preceding specific embodimentsare illustrative ofthe practice of the invention. it is to be understood, however, thatother expedients known to those skilled in the art may be employedwithout departing from the spirit of the invention.

We claim:

1. A method of preparing iron-promoted catalyst in a pelletized form forthe synthesis of ammonia which consists of the,

consecutive steps of 1 fusing a mixture consisting essentially of ironoxides of the approximate composition of magnetite, together withadditional oxides of promoter netals i a t otz il 7 g TABLE VI Activityof the catalyst-percent by weight of NH; at

. Volume of various temperatures-synthesis pressure 260 bars Treatmentof th catalyst of Grain size, catalyst, Space Exumplolll m. cm 3velocity 400 0. 420 C. 440 C. 460 0. 480 C. A-Crude 0. 4-0. 5 2. 5 26,000 1 7 19. 4 21 21. 8 21. 7 P1-Reducedpelletized. 0. 4-0. 5 2. 6 25,000 16. 9 19. 6 21. 2 22. 1 22 D-Unreducedslntered 0. 4-0. 5 2. 6 25,000 10. 5 13. 1 15. 4 17. 3 18. 3 C-Reduced-unpelletized 2-3. 1 6 12,000 12. 1 13. 8 15.4 16. 7 17. 8

BzReduced pelletized reactors which require larger-sized catalysts sincethe production ratio is higher with the 2-3.l-mm. grains obtained fromthe prereduced pellets than from the same-sized grains obtained fromprereduced but unpelletized catalyst (C vs. 3,).

EXAMPLE VIl An oxide composition identical to that of Example Ill wasfused, crushed and reduced under the same conditions as Example lll.However, the treatment to reduce the pyrophoiicity was not performed.The prereduced fragments were then crushed into a powder passing througha l-mm. mesh under an atmosphere of nitrogen. This powder was pelletizedon a ro-; tary press at a pressure of 5,800 kg./cm. under an atmosphereof nitrogen. The pellets were subjected to the mechanical property testsdescribed above. The shearing pressure of the pellets was 13.4 kg./cm.and the percent of dust in the abrasion test was 0.19 percent. Comparingthese results with those of Example ill in Table IV, it can be seen thatthe pelletizing of the reduced material which has not undergone anantipyrophoritic treatment gives a pellet with even better mechanicalproperties. The resistance to shearing is improved b a o Pe en ss.,tb aa lu.rebtwafisfaug proportion of the oxides of promoter metals of fromabout 1 percent to about 10 percent, said oxides of promoter metalsconsisting in part of alumina in an amount of from 0.5 to 9 size ofabout 2 to 10 mm., (4) reducing substantially the fused granules, (5)grinding the reduced fused granules to a particle size not exceeding 2mm., 6) agglomerating the reduced ground product into pellets by simplecompression at a pres sure of from 3 to'l5 tons per cm.*, and (7)recovering said iron-promoted catalysts in a pelletized form.

2. The method of claim 1 wherein said additional oxides of promotermetals are selected from the group consisting of alumina, silica, thealkali metals and earth-alkali metals oxides.

3. The method of claim 1 wherein said additional oxides of promotermetals include from 0.5 to 9 percent alumina and from 0.075 to 2 percentof K 0.

4. The method of claim 1 wherein said additional oxides of promotermetals include from 1 to 2.5 percent alumina and from 0.1 to 0.5 percentof K 5. The method of claim 1 wherein said additional oxides of promotermetals consist of from 1 to 2.5 percent of alumina,

,from 0.1 to 0.5 percent of K 0 and from 0.1 to 0.5 percent of Li 0.

6. The method of claim 1, wherein after step 4, said reduced

2. The method of claim 1 wherein said additional oxides of promotermetals are selected from the group consisting of alumina, silica, thealkali metals and earth-alkali metals oxides.
 3. The method of claim 1wherein said Additional oxides of promoter metals include from 0.5 to 9percent alumina and from 0.075 to 2 percent of K2O.
 4. The method ofclaim 1 wherein said additional oxides of promoter metals include from 1to 2.5 percent alumina and from 0.1 to 0.5 percent of K2).
 5. The methodof claim 1 wherein said additional oxides of promoter metals consist offrom 1 to 2.5 percent of alumina, from 0.1 to 0.5 percent of K2O andfrom 0.1 to 0.5 percent of Li2O.
 6. The method of claim 1, wherein afterstep 4, said reduced granules after cooling are subjected to anantipyrophoric treatment.
 7. The method of claim 1, wherein after step6, said pellets are subjected to an antipyrophoric treatment.